1. Field of the Invention
The preferred embodiments are directed to an apparatus for storing and delivering probe devices for a scanning probe microscope (SPM), and more particularly, a probe cassette for an SPM that is adapted to readily interface with an SPM and includes one or more probe retainers to hold the probe devices under a compressive force without the probe devices sticking thereto.
2. Description of Related Art
Several probe-based instruments monitor the interaction between a cantilever-based probe device and a sample to obtain information concerning one or more characteristics of the sample. The probe devices used by these instruments are typically very expensive to fabricate, and each often costing a thousand dollars or more. They also are quite delicate. As such, great care must be used when handling them, both after fabrication and in preparation for use, as well as when considering shipment, including packaging, and on-site transport options. Prior systems have proven inadequate.
A brief review of these instruments will highlight the challenges associated with maintaining a high yield of usable probes after production. Scanning probe microscopes (SPMs), such as the atomic force microscope (AFM), are instruments which typically use a sharp tip to make a local measurement of one or more properties of a sample. More particularly, SPMs typically characterize the surfaces of such small-scale sample features by monitoring the interaction between the sample and the tip of the associated probe device. By providing relative scanning movement between the tip and the sample, surface characteristic data and other sample-dependent data can be acquired over a particular region of the sample, and a corresponding map of the sample can be generated.
The AFM is a very popular type of SPM. The probe of the typical AFM includes a very small cantilever which is fixed to a support at its base and has a sharp probe tip attached to the opposite, free end. The probe tip is brought very near to or into direct or intermittent contact with a surface of the sample to be examined, and the deflection of the cantilever in response to the probe tip's interaction with the sample is measured with an extremely sensitive deflection detector, often an optical lever system such as described in Hansma et al. U.S. Pat. No. RE 34,489, or some other deflection detector such as an arrangement of strain gauges, capacitance sensors, etc. AFMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid or vacuum by using piezoelectric scanners, optical lever deflection detectors, and very small cantilevers. Because of their resolution and versatility, AFMs are important measurement devices in many diverse fields ranging from semiconductor manufacturing to biological research.
In operation, the probe is most often scanned over a surface using a high-resolution three axis scanner acting on the sample support and/or the probe. The instrument is thus capable of creating relative motion between the probe and the sample while measuring the topography or some other property of the sample as described, for example, in Hansma et al. supra; Elings et al. U.S. Pat. No. 5,266,801; and Elings et al. U.S. Pat. No. 5,412,980.
A typical AFM system is shown schematically in FIG. 1. An AFM 10 employing a probe device 12 including a probe 14 having a cantilever 15 is coupled to an oscillating actuator or drive 16 that is used to drive probe 14, in this case, at or near the probe's resonant frequency. Commonly, an electronic signal is applied from an AC signal source 18 under control of an AFM controller 20 to cause actuator 16 to drive the probe 14 to oscillate, preferably at a free oscillation amplitude Ao. Probe 14 is typically actuated toward and away from sample 22 using a suitable actuator or scanner 24 controlled via feedback by controller 20. Notably, the actuator 16 may be coupled to the scanner 24 and probe 14 but may be formed integrally with the cantilever 15 of probe 14 as part of a self-actuated cantilever/probe. Moreover, though the actuator 24 is shown coupled to the probe 14, the actuator 24 may be employed to move sample 22 in three orthogonal directions as an XYZ actuator.
For use and operation, one or more probes may be loaded into the AFM and the AFM may be equipped to select one of several loaded probes. Typically, the selected probe 14 is oscillated and brought into contact with sample 22 as sample characteristics are monitored by detecting changes in one or more characteristics of the oscillation of probe 14, as described above. In this regard, a deflection detection apparatus 17 is typically employed to direct a beam towards the backside of probe 14, the beam then being reflected towards a detector 26, such as a four quadrant photodetector. As the beam translates across detector 26, appropriate signals are transmitted to controller 20, which processes the signals to determine changes in the oscillation of probe 14. Commonly, controller 20 generates control signals to maintain a constant force between the tip and sample, typically to maintain a setpoint characteristic of the oscillation of probe 14. For example, controller 20 is often used to maintain the oscillation amplitude at a setpoint value, AS, to insure a generally constant force between the tip and sample. Alternatively, a setpoint phase or frequency may be used.
As metrology applications demand greater and greater throughput, improvements to performing conventional AFM measurements, such as that described above, have become necessary. Wafer analysis in the semiconductor industry is one key application. When analyzing such structures at small scales, the corresponding measurements require uniformity control and must be able to accommodate high volume production environments. In this regard, one advancement has been in the area of automated AFMs, which greatly improves the number of samples that may be imaged in a certain time frame by, among other things, minimizing expert user tasks during operation. Instruments for performing automated wafer measurements are varied, but AFM technology offers a unique solution by providing, for example, the ability to perform high-resolution multi-dimension (e.g., 3-D) imaging.
Though automated AFMs provide significant performance advantages by reducing the tasks required by expert users and otherwise streamlining measurements, further improvement is desired. For instance, the manner in which some probe device manufacturers handle and ship probes can create serious challenges in efficiently delivering these often times very costly devices. According to one known delivery method, for example, probe devices for AFMs are delivered in clam-shell packs. Such a clam-shell pack or container 30 including a row 32 of probe devices 34 is shown in FIG. 2. Rows 32 of probe devices 34 are placed, preferably, “tip up” in a base 36 and are covered with a lid 38. In this case, the probe devices 34 are individually loaded into receptacles of the clam-shell pack 30, or mounted otherwise, and then shipped. To do so, the operator typically uses a tweezers to transfer the micromachined or batch fabricated probe devices from the fabrication site to the clam-shell packs. As the lid of the clam-shell pack is closed for shipping, a foam insert, or other holding mechanism, may be included in an attempt to secure the probe devices.
This operation often compromises efficient probe device delivery, for example, by risking operator error through mishandling. The probes can be dropped, scraped or otherwise subjected to unwanted forces that can damage or destroy these delicate and expensive devices. Also, with the probe devices placed in the package “tip-up,” this crucial part of the device is at high risk of becoming damaged. In the end, this manner of handling and shipping probes has clearly been less than ideal.
In addition, not only do loading, shipping and handling probe devices create challenges, the manner in which probe devices are loaded into the customer's AFM, and replaced during operation, can be a challenge as well. Typically, when probe devices are to be loaded into an SPM, the expert user manually transfers the probe devices from the package in which they were delivered and places them in a probe mount of the SPM. The above-noted problems associated with such manual handling of the probe devices apply here as well, with the problems made only worse by the standard type of insert housed by the clam-shell pack that holds the devices, namely, a Gel-Pak® (Gel-Pak® is a registered trademark of Gel-Pak LLC Ltd. of Sunnyvale, Calif.). A Gel-Pak® is an ESD safe container that uses a gel insert 40 (FIG. 2) that engages and holds onto the probe devices, typically, the backs of the probe devices with the tips of the probes normally facing up, as noted previously.
Importantly, as a result, not only does the user need to manually grab the probe devices with a pair of tweezers when loading them, the user needs to turn the probes upside down to place them in the probe mount. To turn a probe device upside-down, the user must often use the tweezers to first grab, and then re-grab the probe with the opposite hand to flip it over, a time consuming process that has a high likelihood of compromising the integrity of the probes (e.g., by mishandling the probes). Alternatively, rather than using two hands, the operator may manually load probes into the AFM by setting the probes down and then picking them up again with the same hand. This procedure clearly creates a slew of other problems mostly directed to potentially damaging the probe, particularly the tip. In either case, this operation is only further complicated by the fact that the probe devices most often have a width and length that are about one millimeter by three millimeters, i.e., they are difficult to handle no matter how careful the operator is when handling the devices.
In the end, given that the probes can cost a thousand dollars each or more, an alternate method of transferring the probes was needed. Ideally, manual handling of the probe devices would be completely avoided.
In one proposed solution disclosed in U.S. Pat. No. 5,705,814, owned by Veeco Instruments Inc., of Santa Barbara, Calif., hereby expressly incorporated by reference herein in its entirety, an automatic tip exchange system is disclosed that uses cassettes loaded with probe devices. In this system, the concept is to load cassettes with probe devices, the cassettes being mountable in an AFM. Also, with this system, the probe devices are shipped, typically, using the Gel-Paks, as described previously, with the customer loading the cassettes upon receipt. When the customer exhausts the probes of a cassette during operation, the customer loads the cassette and simply installs the loaded cassette (or loads the cassette with probe devices after mounting the cassette in the AFM) in a staging area accessible by the AFM. This is shown in the AFM 40 of FIG. 3. In this case, a probe cassette 41 holding one or more probe devices 42 is positioned on an X-Y translation stage 43. A vacuum-based probe mount 45 includes a vacuum mechanism 44 for securing probe devices to mount 45 during AFM operation. Probe mount 45 is supported by a scanner 46 via an oscillator 47 that may be used to oscillate the selected probe device 42 mounted thereto. Scanner 46 is typically a piezoelectric tube-type scanner, or a piezoelectric flexure-based scanner. In operation, probe mount 45 can be manipulated to select a probe device 42 and position the selected probe device for measuring a sample (not shown). Importantly, using this system, the user can continue to make AFM measurements without manually replacing individual probe devices 42 each time the operator wishes to use a new probe device, i.e., each new probe device being selected from probe cassette 41 disposed on stage 43 can be automatically loaded onto probe mount 45.
In view of the above, even with this automatic select/load system, the expert operator must still manually manipulate the probe devices at the AFM site. Notably, cassettes loaded by the probe device manufacturer are not employed with this and other known systems because the probe devices too often do not remain housed in the package, particularly when the AFM operator removes the lid. For example, some probes will typically stick to the lid of the package. Some AFM users therefore have found it more cost-effective to engage in the laborious and inefficient task of unloading the conventional shipping package (e.g., a Gel-Pak) manually.
Another problem with this and other known delivery and probe loading arrangements has been that the probe devices loaded in the packages can move within the package, especially if jarred, e.g., after being dropped. This clearly increases the risk that the probes might be destroyed or otherwise have their performance altered. In the end, all of these challenges with known probe device delivery and loading arrangements create significant problems with respect to compromising the yield of fully operational probes.
With further reference to one of these challenges, namely probe devices sticking to the lid or other surrounding surfaces, such sticking is often due to the use of a plastic cover when shipping the probe devices. Such plastic covers create significant static charge that attracts probe device (electrostatic discharge—ESD) causing the probe devices to stick thereto. As a result, ESD safe containers are preferred, most often including a conducting metal holder that prevents the probe devices from sticking to the lid. However, such metal holders are not immune from probe devices sticking thereto. Moreover, the use of such metal holders has the additional disadvantage that they oftentimes are unable to absorb significant impact forces, for instance, due to dropping of the package. Again, considering that probe devices can cost a thousand dollars or more, known probe delivery and loading arrangements have been found to be non-ideal.
As a result, the field of scanning probe microscopy, such as automated AFM operation (e.g., for use in the semiconductor industry), was in need of a system and method able to readily exchange probe devices from a package in which they were shipped to the AFM, while also improving yield of usable probes. In particular, a method and apparatus for delivering and replacing probe devices was needed in which the probe devices are maintained in a secure package able to absorb impact yet not damage the probe devices, while also insuring that the probe devices do not stick to any part of the package once the user receives the probe devices and wishes to introduce them to the AFM.